A component of older automobiles, race cars and lawn-care vehicles, a carburettor provides a mixture of gasoline and air to a vehicle's engine for acceleration. When the carburettor's air and gasoline mixture is not balanced and too much air is being sent to the engine, it is known as a lean mixture. A lean mixture has a number of symptoms and is the opposite of a rich mixture -- when the carburettor is sending too much gasoline to the engine.
One symptom of a lean carburettor is hesitation when you give the vehicle gas. The vehicle may shake, move forward slowly and require accelerator pumping to send more gasoline to the carburettor. The extra gasoline overcompensates for the extra air being sent to the engine. The problem could be a dirty carburettor, faulty rubber diaphragm on the accelerator pump or a bad rubber cup/seal on a piston.
Engine Stalling at Stops
The second symptom of a lean carburettor is stalling upon slowing or stopping. The vehicle may start shaking at first, then stall, requiring a restart and the pumping of the accelerator. This problem comes from a carburettor being out of adjustment, internal air leaks or a sticky needle valve that keeps gasoline from entering the carburettor.
Engine Idling Improperly
The third symptom of a lean carburettor is a rough idle, such as an engine that races and then slows down. The engine may shake and possibly stall if the idle becomes too low. The problem is due to a dirty idle mixture circuit, a dirty carburettor, an incorrect idle mixture adjustment or a failing carburettor.
Combination of Issues
These symptoms may present themselves individually or all at once. Your vehicle may require the replacement or the adjustment of multiple components, including but not limited to, the carburettor, electrical system, emission system, engine valves and other engine components.